Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of the target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target race-way.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known dispositions in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to a support by use of soldering, brazing, diffusion bonding, clamping, epoxy cements, or with interlocking annular members.
For example, the prior art shown in the Hillman Pat. No. 4,885,075 discloses a cooling device for a sputter target and source utilizing an annular shape member of high thermal conductivity disposed between the cathode and the target electrode. This annular member is constructed of a base that is disposed in a corresponding recess in the cathode and a member projecting perpendicularly from the base disposed in a corresponding annular-shaped recess in the target electrode. Upon heating of the target, the target electrode expands radially against the members, thereby reducing the temperature of the target electrode.
A further example of the prior art is the Lamont Pat. No. 4,457,825 which discloses a sputtering assembly wherein a circular cathode ring surrounds a centrally located circular anode. The cathode ring as a sputter surface having a generally inverted conical configuration. The target is cooled by thermal contact between an outer rim of the target and a cooling wall disposed along the periphery of the outer rim of the target.
Other examples of cathodic sputtering target cooling assemblies include U.S. Pat. Nos. 4,060,470 (Clarke); 4,100,055 (Rainey); and 4,564,435 (Wickersham). In these types of structures, when the sputter target is at ambient temperatures, the target is slightly smaller in diameter than the cooling wall, and the target may move freely in the axial direction for easy insertion of same into the assembly. When the sputter target is in operation and heated, the target expands into close physical contact with the cooling wall. As a variation to this approach, U.S. Pat. No. 4,855,033 (Hurwitt) provides intermeshing projections and recesses formed in the target sidewall and surrounding cooling wall to improve heat transfer. Further, the target comprises an arch-shaped convex back portion so that plastic deformation of the target during sputtering will translate into a radially directed vector to urge the target sidewall into contact with the surrounding cooling wall.
One problem experienced with the prior art sputter target/backing plate assemblies taught in Hillman is that, during sputter usage, the backing plates warp or bow outwardly at the center, most probably due to cooling fluid pressure exerted along the underside portion of the backing plate, causing disengagement of the projections and recesses of the coupling means. This, in turn, severely retards heat transfer, leading to target deterioration and failure.
Accordingly, it is an object of the invention to provide a readily mounted mating target/backing plate structure that accommodates this bowing tendency, thus acting to maintain coupling of the target-backing plate. The ideal combination would employ a means for providing an easy change from one target to the next, while ensuring efficient thermal contact between the backing plate and target.